10 



MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



made on the osmotic pressure of the cells of trees, and any discussion of 

 the water relations of trees must consider this phase of the subject. 



Since the freezing point of a liquid varies with its osmotic value, the 

 determination of the freezing point of plant saps and juices is a com- 

 mon method of determining the osmotic value of the plant liquids. 

 Thus Dixon and Atkins (49, 51) measured the osmotic value of sap 

 that had been centrifuged from the sapwood of trees and obtained the 

 following results for Ulmus campestris L. on March 4, showing that 

 the osmotic value varies with the height in the tree: 



u . , , , ,, , , , v Osmotic pressure 



Height above the ground (meters) : (in atmospheres) 



0.6 1. 13 



16.0 2. 09 



20.0 3. 52 



Similar results showing both the effect of the height in the tree and of 

 the season are given in table 2 for Acer macrophyllum Pursh. 



Table 2. — Variation in osmotic -pressure with height in tree and season 



Height (meters) 



Osmotic pressure 



Oct. 13 



Feb. 25 



Apr. 14 



Root 



Atmos- 

 pheres 

 0.72 



.63 

 .56 

 .42 



Atmos- 

 pheres 

 1.07 



1.76 

 1.76 

 2.14 



Atmos- 

 pheres 

 1.34 



1.31 

 1.30 

 1.30 



Stem at— 



Ground 



2.0 



4.0- 





Height (meters) 



Stem at— Con 



6.0 



8.0 



10.0 



Osmotic pressure 



Oct. 13 Feb. 25 Apr. 14 



Atmos- 

 pheres 

 .49 

 .57 

 .81 



Atmos- 

 pheres 

 2.15 

 2.70 

 3.71 



Atmos- 

 pheres 

 1.73 

 1.98 

 2.16 



Similar results were obtained also with Cotoneaster jrigida Wall., 

 Ilex aquijolium L., and other species, although the osmotic pressure 

 was found to vary somewhat with the species. The sap was centri- 

 fuged from the sapwood, but the centrifuging force is not given. The 

 authors, however, call attention to the amount of liquid that can be 

 obtained in this fashion and state that in December a piece of Salix 

 babylonica L. wood 2 cm in diameter and 10 cm long gave as much as 

 4 cc of water, although a yield of 1 to 2.5 cc was the usual amount for 

 pieces this size. 



These workers were among the first to note the gradient of the os- 

 motic pressure in the cells of the wood as height above the ground 

 increases. The importance of this for the ascent of sap and the life 

 processes of trees will be discussed further in the following section. 



WATER CONTENT OF LEAVES 



Since the cause of the ascent of sap in trees is very complicated and 

 lies somewhat outside the scope of this work, it will be touched upon 

 only very briefly in connection with the other problems involved. 

 Briefly, Dixon and his coworkers have shown that the water is pulled 

 up the tree by the leaves during transpiration, radiation (from the 

 sun) supplying the necessary energy. As the water evaporates from 

 the living cells of the leaf, more water is drawn in from the tracheids 

 and other conducting cells in the leaves, stems, and roots, aided chiefly 



